External fixation systems use bone pins or wires to stabilize bone segments. These pins or wires are held in space relative to each other by a fixation frame. The frame can be made up of rings, struts, bars or other structural members. The simplest frame is usually made up of bars (sometimes referred to as rods). To hold the bars to each other and to the fixation elements, typically a clamp is engaged to two or more elements. Clamps may also be employed to hold bars to rings, pins to rings, struts to rings, etc.
The user typically constructs the external fixation system by placing the pins into the bone and assembling the frame into the approximate shape needed, then adjusting the position of the bone to get the bone fragments in their proper anatomic location. Then the frame is locked into place by tightening the clamps and other adjustment means.
U.S. Pat. No. 5,752,954 to Mata describes a clamp used to create a frame where the bars are snapped into the clamp. The clamp is constructed of two jaws, each with opposing channels the relative shape of half of the element they are grabbing (such as a bar or pin). The jaws are biased towards each other along the clamping axis by a spring element. When the bar or pin is snapped into the jaw, the biasing element holds the jaws against the bar or pin. This makes assembly easy, but a common complaint is that the jaws can also allow for the bar or pin to pop out when the assembly is being adjusted to the appropriate position.
U.S. Pat. No. 7,004,943 to Ferrante describes a clamp mechanism with two jaws that slide relative to each other perpendicular to the clamping axis. The direction of opening of the jaws is such that force applied by the bar or pin against the jaw does not cause the jaw to open up, which reduces the likelihood of pop-out.
Both of these devices as described, and other similar devices, have the jaws sized to match the component that they lock to, either the bar, pin or another element. One enhancement to the Mata design incorporates a jaw geometry that can clamp against pins of various sizes because the biasing element pushes the jaws together, taking up any slack due to differential sizes. Pop-out can be reduced by adjusting the clamping means to tighten the jaws to nearly full clamping, but making this adjustment is inconvenient for the user.
However, the jaw designs of these prior clamps either do not provide for provisional locking or are limited to provisional locking of only one size of fixation element.